The present invention relates to microelectromechanical devices and, more particularly, to micromachined devices containing moving parts supported by flexible members or springs.
Microelectromechanical systems (MEMS) are being developed as alternatives to conventional electromechanical devices such as relays, actuators, valves and sensors. MEMS devices are potentially low cost devices, due to the use of microfabrication techniques. New functionality may also be provided because MEMS devices can be much smaller than conventional electromechanical devices.
Many potential applications for MEMS devices involve moving parts. For example, sensors, switches, valves and positioners use MEMS devices as actuators for movement. If properly designed, MEMS actuators can produce useful forces and displacement while consuming reasonable amounts of energy. The moving parts of these devices are supported on flexible members, such as flexures or springs. The properties of these springs often are critical to the performance of the overall system.
The movable or flexible elements of a MEMS device typically are required to move repetitively from a first, typically resting, position to a second position under application of a force. The flexible element should be able to return to its initial position once that force is removed (material memory or low hysteresis), or upon application of an opposing force. Thus, it is desirable that the flexible member exhibit high fracture toughness, high yield strength and low plasticity so that it can undergo many cycles without material failure. In addition, many applications require that the MEMS flexible element be a conductive member, as well as a mechanical or structural member. Lastly, it is desirable that the material exhibit good corrosion resistance, in particular to the etchants used in the processing of the device, and low toxicity.
MEMS or micromechanical devices have been built using silicon, polysilicon, metals and organic polymers. However, none of the currently available materials satisfy all of the performance requirements for a MEMS device. Many of these materials exhibit properties such as brittleness, fatigue, creep or plastic deformation that are undesirable or unacceptable to device performance.
Silicon has been used in MEMS devices as a flexible element, however, its brittleness and relatively poor conductivity make it impractical in many applications. Even highly doped silicon is not suitable for applications where currents of several milliamperes must be passed through the flexible element with low voltage drop. Furthermore, silicon is typically deposited at high temperatures, making it incompatible with many other microfabrication steps.
Pure metals such as gold, nickel and copper, have been used, however, they exhibit poor yield strength and plastic deformation. Precipitate hardened metals have been investigated, however the dispersion of microprecipitates limits fatigue strength, and requires heat treatment and mechanical working to achieve the desired microstructure.
Beryllium-doped copper (Be—Cu) is a good spring material, however, it is highly toxic and subject to corrosion. The corrosion films normally observed on copper surfaces may actually be thicker than the desired MEMS spring or flexible member. In addition, many of the chemical etches used in the semiconductor process unacceptably etch the copper alloy.
Thus, there is a need for flexible members that overcome the performance limitations of flexible members currently used in MEMS devices.
There is a further need in many applications that the MEMS device, and the flexible member of the MEMS device, be electrically conductive.
There is a further need in some applications for flexible MEMS elements that are non-ferromagnetic.
There is a further need for materials used for the flexible member of the MEMS device that are compatible with other microfabrication processes used in the preparation and processing of the MEMS device.